The preparation of stable, nonpolar organic solvent dispersions of polymer particles insoluble in non-polar organic solvents usually involves formation of these insoluble particles in the presence of a polymeric stabilizer soluble in non-polar solvents, such that the insoluble polymer particles become grafted to the stabilizer backbone. Typically, the insoluble polymer particles are formed by the polymerization of soluble ethylenically unsaturated monomers dissolved in the non-polar solvent. See Keith E. J. Barrett, Dispersion Polymerization in Organic Media (1975).
The stabilizer thus provides a lyophilic layer of soluble polymer anchored to the surface of the insoluble polymer particles. This layer provides solvation of the insoluble particles and prevents the particles from approaching each other so that a sterically stabilized colloidal dispersion is achieved. Strong anchoring of the solvated moiety to the insoluble particle is essential to prevent either desorption from the particle surface or displacement during particle collision.
The stabilizer may be a saturated polymer, in which case the insoluble particles are typically grafted onto the saturated stabilizer backbone by the use of a free radical initiator such as benzoylperoxide. The function of the initiator is to generate reactive sites on the saturated stabilizer molecule, by hydrogen abstraction, which subsequently initiate the graft polymerization of the monomeric precursors of the insoluble polymer particles at these reactive sites. Although this reaction is useful in the preparation of graft copolymers, it is often seriously limited by the reluctance of the saturated backbone to undergo free radical attack and results in a random and haphazard generation of reactive sites.
A preferred method, rather than this random and haphazard generation of reactive sites on the stabilizer backbone, is to construct the stabilizer backbone molecule in such a way as to produce ethylenically unsaturated pendant moieties on the backbone to serve as sites to be activated by the initiator for the in situ graft polymerization of the insoluble polymer particles.
Many different reactions can be utilized to introduce these ethylenically unsaturated pendant groups into the stabilizer backbone. Typically, a copolymer stabilizer precursor is formed wherein at least one of the comonomers has an unsaturated bond and a reactive group. The unsaturated bond is reacted in the copolymerization but the reactive group remains intact. The copolymer stabilizer precursor thus has pendant reactive group containing moieties. The reactive group containing moieties of the stabilizer precursor are then further reacted, with a monomer specifically selected to be reactive with the reactive group and to provide an ethylenically unsaturated site for subsequent activation and graft polymerization, to form the stabilizer backbone. Pairs of monomers which may be reacted to provide these ethylenically unsaturated pendant moieties are disclosed in U.S. Pat. No. 3,900,412. Either monomer of the pair may be copolymerized into the stabilizer precursor.
The reaction between most of these monomer pairs, however, takes place with some difficulty, especially in aliphatic hydrocarbon solvents. For example, it is usually necessary to maintain the reactants at temperatures of from about 100.degree. to 150.degree. C. for about 8 to 12 hours, in the presence of a tertiary amine catalyst, to convert about 25 percent of the pendant reactive groups of the stabilizer precursor into the pendant ethylenically unsaturated groups of the stabilizer. Furthermore, the reaction between these monomer pairs typically must take place in the presence of a polymerization inhibitor in order to avoid the further polymerization of the pendant ethylenically unsaturated moieties being formed with unreacted monomer in the solution.